Spark plug



Patentedl Jan. 97,., 194@ PATENT OFFICE SPARK PLUG Douglas Haynes Corbin, Dunstable, England, as-

signor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application May 27, 1938, Serial No. 210,482 In Great Britain June 2, 1937 4 Claims.

This invention relates to spark plugs.

The object of the invention is to provide an improved spark plug which will conform to the requirements of modern internal combustion en- 5 gines. Such requirements comprise the ability to withstand high temperatures, high compressions, and the corrosive anti-knock compounds.

A spark plug according to the invention the scope of which is indicated in the appended l claims, comprises an insulator formed of a refractory oxide, 4and an outer shell of a non-expansible metal, the insulator and shell being tapered outwardly over their contacting surfaces, that is to say tapered outwardly from the electrode end l of the plug which is positioned inside the combustion chamber.

Preferably only a part of the shell is made of non-expansible metal since it is machinable only with difficulty: the remaining part can be of normal machinable metal.

Non-expansible is of course -a relative and not an absolute term and is intended to define metals which have a very low coeflicient of expansion or one near to that of the refractory oxide insulator. A nickel-cobalt steel containing 28% nickel and 18% cobalt has been found the most suitable.

There may be interposed, if desired, between the tapered surfaces of the insulator and shell, a

thin coating of a soft metal such as copper or silver.

Figures 1 and 2 of the accompanying drawing show in part section, two exemplary embodiments of the invention.

In the figures, like parts have the same reference numerals.

The spark plug shown in Figure 1 comprises a central electrode I, which is screwed into a shouldered insulator 2 and is connected at its top end by means of a tension spring 3 to screw-threaded stud 4 to which the outside lead (not shown) is `attached in any convenient manner. An outer insulator sleeve 5 lies within a two-part outer shell 6, l, between the upper part 6 of which and the sleeve 5 is a mica sleeve 8.

The upper shell part 6 is formed of any material usually used for this purpose such as mild steel or bronze. The lower shell part 'I however, is formed of a non-expansible metal such as a nickel cobalt alloy steel, and the two shell parts n arebrazed together to form a shoulder, as shown,

on which the outer insulating sleeve 5 rests,

As indicated in Figure l the inner insulator 2, which is preferably formed of a highly refractory 5 oxide, and the lower shell part l. are formed with (Cl. 12S-169) a. slight outward taper such that gas pressure inside the combustion chamber will force the insulator tightly into the shell. Strain due to the difference in coeiilcients of expansion of the insulator and the shell can be relieved by a thin 5 intermediate lining 9 of soft metal such as silver or copper which may be deposited on either the insulator or shell. Such a lining can be omitted if not found necessary.

The formof the sparking electrodes is imma- 10 terial to this invention, and is shown in the draw ing purely diagrammatically.

The embodiment shown in Figure 2 differs from that shown in Figure 1 in several respects.

The central electrode I instead of being held at its 15 threaded portion by the tension spring 3, is engaged by an internally threaded cap I0 which seats on the top of the insulator 2 and which has its threaded portion inside the enlarged bore of the insulator so as to engage the threaded part 20 of the electrode.

The inner insulator 5 shown' in Figure 1 is omitted in Figure 2 so that there is a space between the top part of the central insulator 2 and the inner wall of the mica or glass insulator 25 sleeve 8. Y

The shell of the plug shown in Figure 2 has two outer parts 6a 6b made of the ordinary mild steel or bronze. The upper part 6a screws into the lower part as shown. The part of the shell made $0 of non-expansible metal however, is made as a separate, internal sleeve l and as before its inner wall ,is outwardly tapered so asjto fit to the outer outwardly tapered surface of the insulator 2. The optional soft metal lining is indicated at 9. The sleeve 'l furthermore is formed with a top shoulder as shown and between this and the lower end of the upper shell part 6a is inserted a spring steel washer II. The insulator itself is shouldered to fit on to a gasket I2 on a ledge on 40 the lower shell member 6b.

Thus with this construction the insulator-electrode assembly is first fitted into the sleeve 1. The two parts are then inserted in the lower part of the Ashell 5b so as to rest on the gasket I2. Then the spring steel washer Il is placed on the top shoulder of the sleeve 1, and the upper shell part 6a is screwed in so that it clamps tightly on the washer I I and holds the sleeve between it and the gasket I2. The remainder of the plug is then 50 assembled in the usual way.

T'here is thus a sealed, gas-tight connection between .the insulator 2 and the sleeve 'l by virtue of the outward taper, and a similar connection between the sleeve 'I and the shell part 6b by virtue of the washer H and gasket l2. The shoulder `on the upperpart ofthe sleeve I strengthens it against outward expansion pressure of the insulator and at the same time serves as a means for securing the insulator-sleeve unit in place.

YA spark plug 'according to theinvention is particularly useful in high-compression-ratio and high-speed engines, for the refractory oxide insulator can withstand the high temperatures, and is not affected by anti-knock compounds; and the itbetween the insulator and the shell is such asf to prevent escape of gas.

It is to be understood that the invention may be carred into eifect in ways other than that above artlcularly described, within the scope of the appended claims.

I claim:

1. A spark plug comprising an insulator formed of a sintered refractory oxide, an outer shell in two parts, and a sleeve of substantially nonexpansible steel withinv one of said parts and 4having a shoulder engaged by the other of said parts when the two parts are assembled, the sleeve and insulator being mutually engaged over outwardly tapering surfaces.

2. A spark plug comprising an insulator formed of'a sintered refractory oxide, a shell having upper and lower parts in screw-threaded relation,

a ledge in the lower shell part and a sleeve of substantially non-expansible steel tltting on said ledge and having at its upper end a shoulder, and a spring washer between said shoulder 'and the 'lower end of said upper shell part, said sleeve andl insulator being in mutual engagement over outwardly tapered surfaces.

3. A spark plug for internal combustion engines comprising a hollow tubular shell made up of an inner part of substantially non-expansible steel which is heated by hotgases within the combustion chamber, and a second part of ordinary steel, said parts being joined together to provide a unitary shell structure and the interior of said inner part being tapering, with the smaller end arranged outward relative to the combustion chamber; and a refractory oxide insulator having a tapered portion seating within the tapered interior of said inner shell part, said insulator being forced into engagement with the tapered inner surface of said inner shell part by the gases within the combustion chamber to thereby provide a gas-tight seal between the insulator andV the shell.

4. A spark plug according toA claim 3, having a thin layer of comparatively soft metal between the insulator and the substantially non-expan sible steel part of said hollowtubular shell.

DOUGLAS HAYNES CORBIN. 

